This invention relates to fiber-reinforced porous thermoplastic sheets and, more particularly, to such sheets for use in the production of fiber-reinforced thermoplastic resin articles.
Thermoplastic sheet material is widely used in manufacturing molded articles. These sheets are frequently reinforced with fiber to increase the strength and rigidity for many applications. One such sheet material is described in U.S. Pat. No. 4,734,321. This patent describes an air permeable sheet-like structure comprised of reinforcing fibers bonded with unconsolidated particulate plastic material for use in molding articles. Prior to molding, this porous sheet may be preheated by hot air permeation for homogeneous heating of the whole structure. A process for forming permeable sheet material useful for molding is described in U.S. Pat. No. 4,882,114. In this patent the porous sheet is made by laying down and draining a foamed aqueous dispersion of discrete fibers and unconsolidated thermoplastic or thermosetting particulate and drying the formed web. The resultant web is preheated and compression molded to form a shaped fiber reinforced plastic article. During the preheating step prior to molding, enough heat must be provided to melt the plastic particulates so that they will flow during the molding step. In a hot air convection oven, the heat delivered to the composite porous sheet is dependent upon the mass flow of heated air through the medium being heated and mass flow depends upon heating time and the pressure drop across the sheet. These last two parameters are inversely related; a low pressure drop requires a long heating time and to achieve short heating times a large pressure differential is required. The design of the heating oven becomes far more complex and expensive when high pressure differentials are required. For economic reasons, it is desirable to keep the required pressure differential below 10" H.sub.2 O, preferably below 6" H.sub.2 O. To attain the appropriate heat delivery to the sheet then, a longer heating time is necessary. However, long heating time is similarly undesirable because exposing polymers to elevated temperatures promotes polymer degradation. Degradation is a time at temperature relationship. To avoid degradation it is important to minimize the time the polymer is exposed to high temperatures.